The present invention relates to an SO.sub.2 gas sensor for measuring the sulfur dioxide (SO.sub.2) gas concentration in the exhaust gas of a combustion engine, or the like, or in the air. Particularly, the present invention relates to a SO.sub.2 gas sensor which can reduce the influence of coexistent oxygen (O.sup.2) on a value obtained by a SO.sub.2 gas measurement and which can operate even at such a high temperatures as 600.degree. C. 900.degree. C.
In boilers for thermal power generation or incineration facilities, there are emission standards on toxic gases that occur in the exhaust gas, such as NO.sub.x and SO.sub.2, which exist for environment protection. Each facility is required to monitor the concentration of these toxic gases in order to prove that the standards are being followed. In thermoelectric power plants or incineration facilities, there is used a measuring apparatus of a nondispersive infrared ray absorption type (NDIR type) to monitor these air-pollutive gases. Since the measuring apparatus is not directly inserted into the exhaust gas, the exhaust gas is sampled by an absorption pump and analyzed in a place separate from the passage for the exhaust gas.
However, in the NDIR type of measuring apparatus, a sampling apparatus is exposed to high temperatures. Therefore, it requires rather frequent maintenance checks, which because of various restrictions are difficult to carry out without stopping the operation of the boiler or the incineration facilities.
Further, the apparatus itself must have gas-pretreatment portions for removing dust and water contained in an exhaust gas, which in combination with the use of an absorption pump, inevitably enlarges the apparatus and raises its price.
Furthermore, because a measurement of the concentration of toxic gases such as SO.sub.2 in an exhaust gas requires the absorption step by the use of an absorption pump, even when such a concentration in an exhaust gas reaches nearly a critical level by, for example, an unexpected extraordinariness of combustion facilities; it is difficult to avoid delay in a response time, and a certain time lag is necessary to cope with the extraordinariness, or the like, which increases the risk of an unexpected accident.
Additionally, since the sensor used in the apparatus sustains interference of CO.sub.2, hydrocarbons inevitably discharged into an exhaust gas, or the like, a precise measurement cannot be expected.
The other methods for measurement shown in JIS B7981 are (1) an electrolytic conductivity method, (2) an ultraviolet ray absorption method, and (3) controlled potential electrolysis. However, these methods have problems regarding sampling due to the aforementioned nondispersive infrared ray absorption, and each of these methods is influenced by peculiar interferential gases.